Pulse energy of a UV laser depends on its emission duration and its excitation duration. Accordingly, higher pulse energy can be generally obtained as the pulse repetition frequency is lower, while the pulse energy becomes lower as the pulse repetition frequency increases.
The peak output and the pulse width depend on the pulse repetition frequency, the kind and the length of the crystal generating the fundamental wave, and the LD (laser diode) output exciting the crystal. Accordingly, the higher the pulse repetition frequency is, the longer the emission duration (that is, the pulse width) is. When the pulse repetition frequency is low, there is a fear that the crystal is damaged. Therefore, the crystal is protected to generate no wave in the low frequency range, and the maximum output frequency is adjusted to be slightly higher than the lower limit frequency. The peak output is lowered gradually with increase of the frequency.
FIG. 10 is a graph showing the output characteristic of a typical high-peak short-pulse-width YVO4 UV laser applied to machining printed circuit boards.
The YVO4 UV laser has an external cavity structure containing a fundamental wave generation crystal and a wavelength conversion crystal for the third harmonic generation (THG). When a Q switch (Q-SW) is turned on, a fundamental wave whose wavelength is 1,064 nm is generated, and the wavelength is converted at the same time. Since an electro-optic device of polarization modulation type having a high response speed is used as the Q-SW, there hardly appears a residual fundamental wave, and the pulse width is narrow. However, the peak intensity is so high that the crystal is apt to be damaged. Therefore, the position of the wavelength conversion crystal is shifted periodically so that the device life is secured.
As shown in FIG. 10, the maximum average output is about 11 W at the pulse frequency 40 kHz. However, the average output becomes lower with increase of the frequency, and it reaches about 5.5 W at 100 kHz. When the frequency is increased, there is a small change in the pulse leading edge, but the pulse width increases from 25 ns to 35 ns in the pulse trailing edge with a gradual slope. The pulse energy is reduced to 25% at 100 kHz in comparison with that at 50 kHz, and the peak output is reduced to 19%.
FIG. 11 is a graph showing the output characteristic of a typical low-peak long-pulse-width YAG UV laser applied to machining printed boards in the same manner as the YVO4 UV laser. For the sake of comparison, the pulse width of the YVO4 UV laser shown in FIG. 10 is illustrated by the broken line.
The YAG UV laser has an internal cavity structure containing a fundamental wave generation crystal and a THG wavelength conversion crystal. When a Q-SW is turned on, a fundamental wave is generated, and the wavelength is converted at the same time. Since an acousto-optic device having a low response speed is used as the Q-SW, the fundamental wave keeps passing through the conversion crystal even after the Q-SW is turned off and till the passage of the beam is terminated. Therefore, the output duration, that is, the pulse width is long, but the peak intensity is so low that the damage to the crystal can be reduced.
As shown in FIG. 11, the maximum average output is about 11 W at the pulse frequency 40 kHz. However, the average output becomes lower with increase of the frequency, and it reaches about 5.5 W at 100 kHz. When the frequency is increased, there is a small change in the pulse leading edge, but the pulse width increases from 130 ns to 180 ns in the pulse trailing edge with a steep slope. The pulse energy is reduced to 25% at 100 kHz in comparison with that at 50 kHz, and the peak output is reduced to 15%.
When FIGS. 10 and 11 are compared with each other, the average output is substantially identical between the both in a range of the pulse frequency 40–100 kHz. However, when the peak output at the frequency 50 kHz in the YVO4 UV laser is regarded as reference (100%), the peak output at the frequency 50 kHz in the YAG UV laser is 20%, and the peak output at the frequency 100 kHz is about 4%.
As is apparent from FIGS. 10 and 11, the pulse width of the conventional lasers depends on the frequency. In the case of the YVO4 UV laser, the pulse widths are limited in the range of 20–35 ns, and in the case of the YAG UV laser, the pulse widths are limited in the range of 130–180 ns. It is therefore impossible to perform machining with pulse widths in the range suitable for machining insulating layers, which range is shown by the shaded portion in FIG. 11.
The following relations are established among the average output, the peak output, the pulse energy, the pulse frequency, the pulse width, the material removal quantity, the energy density and the output density.
pulse energy (J)=average output (W)/pulse frequency (Hz)
peak output (W)=pulse energy (J)/pulse width (s)
removal quantity ∝pulse energy/number of pulses
energy density (J/cm2)=pulse energy (J)/beam area (cm2)
output density (W/cm2)=peak output (W)/beam area (cm2)
That is, the pulse energy is in inverse proportion to the pulse frequency, and the peak output is in inverse proportion to the pulse width. Typically the output density (energy density per unit time) is much larger than decomposition thresholds of materials except metal materials, such as copper, having high thermal conductivities. Therefore, the removal quantity is substantially in proportion to the total energy (pulse energy×number of pulses). Accordingly, when machining is performed near the maximum output, the efficiency can be improved, and the machining speed can be increased.
On the other hand, in the case of an FR-4 material which is a typical printed circuit board material, the necessary energy density and the energy density threshold of a copper conductor layer is about 10 times larger than that of an insulating layer (layer of glass fiber impregnated with resin) (copper:glass fiber:resin≅10:3:1) due to differences in material properties. In addition, there are optimal conditions for each material. Specific description will be made below.    (1) When the energy density and the output density are too high in the case where the copper conductor layer on the surface is machined, the insulating layer just under the copper conductor layer is machined with the high power as soon as the conductor layer is removed. Thus, the resin is removed, so that the large undercut is formed in the insulating layer. On the contrary, when the energy density and the output density are too low, supplied heat diffuses to the periphery of the portion to be machined, so that the removal quantity per pulse is reduced. Therefore, the number of pulses increases, so that the machining speed is slowed down. From above, the energy density and the output density suitable for machining the copper conductor layer are in the range of about 5–10 J/cm2 and in the range of about 150–300 MW/cm2 respectively in the case of a high-peak short-pulse-width laser, and in the range of about 10–20 J/cm2 and in the range of about 100–200 MW/cm2 respectively in the case of a low-peak long-pulse-width laser.    (2) When the energy density and the output density are too high in the case where the impregnated glass fiber is machined, the resin around the glass fiber is removed by surface reflected light, so that the projecting length of the glass fiber from the resin surface is increased. In addition, when the remaining amount of the insulating layer is reduced due to the progress of machining, the copper conductor layer at the hole bottom is injured. On the contrary, when the energy density and the output density are too low, the removal quantity per pulse is reduced, so that the number of pulses increases. Thus, the machining speed is slowed down.
From above, the energy density and the output density suitable for machining the glass fiber are in the range of about 2–6 J/cm2 and in the range of about 100–200 MW/cm2 respectively in the case of a high-peak short-pulse-width laser, and in the range of about 3–8 J/cm2 and in the range of about 60–120 MW/cm2 respectively in the case of a low-peak long-pulse-width laser    (3) When the energy density and the output density are too high in the case where the resin is machined, the copper conductor layer at the hole bottom is injured. On the contrary, when the energy density and the output density are too low, the residue of the resin at the hole bottom is increased. In addition, the number of pulses increases so that the machining speed is slowed down.
From above, the energy density and the output density suitable for machining the resin are in the range of about 0.5–1.5 J/cm2 and in the range of about 15–30 MW/cm2 respectively in the case of a high-peak short-pulse-width laser, and in the range of about 0.7–1.5 J/cm2 and in the range of about 10–20 MW/cm2 respectively in the case of a low-peak long-pulse-width laser.
As is apparent from the aforementioned description, based on the hole quality, the hole shape and the machining speed, there are an upper limit and a lower limit in thresholds of the energy density and the output density suitable for each material to be machined. When machining is performed near the maximum output, the hole quality or the hole shape deteriorates. For example, when a hole having a diameter of 50 μm is machined in the copper conductor layer, the glass fiber and the resin using the UV lasers shown in FIGS. 10 and 11, practical pulse frequencies are limited to a range of about 40 Hz or lower, a range of about 60 kHz or lower and a range of about 100 kHz or lower respectively in term of suitable energy density and suitable output density.
Therefore, in the background art, machining is performed while changing the pulse frequency in a range of from 40 kHz to 100 kHz on the basis of energy (or peak output) required for machining the copper conductor layer, the glass fiber and the resin with the progress of machining (that is, whenever the material to be machined is changed). Alternatively, machining is performed while determining the pulse frequency on the basis of energy (or peak output) required for machining the copper conductor layer, and changing the LD output (that is, the output of a laser oscillator) for the glass fiber and the resin without changing the pulse frequency, so as to machine them with the adjusted energy (or peak output).
In order to stabilize the output of the UV laser, it is necessary to control the temperature of the wavelength conversion crystal within 0.1° C. That is, either when the pulse frequency is changed or when the peak output is controlled by the LD output, if an SHG or THG wavelength conversion crystal (LBO, CLBO or BBO) is off thermal balance, the beam outgoing angle will fluctuate due to fluctuation in refractive index caused by fluctuation in crystal temperature. As a result, the hole position accuracy or the hole shape will deteriorate.
However, even when the temperature of the wavelength conversion crystal was controlled within 0.1° C., there occurred a fluctuation in beam outgoing angle of, for example, 40 μrad at a frequency of 60 kHz and 60 μrad at a frequency of 80 kHz with respect to that at a frequency of 40 kHz due to a time lag or the like. As a result, the hole position accuracy deteriorated up to about 5 μm. In addition, the hole quality and the hole shape deteriorated due to a fluctuation in output and a broken beam mode (spatial energy distribution).
Further, since conditions suitable for each material cannot be secured, desired hole position accuracy, desired hole quality and desired hole shape could not be obtained.
Therefore, for example, JP-A-2002-335063 discloses a machining method in which the pulse frequency is fixed, while the peak output is changed over outside a laser oscillator so as to machine a copper conductor layer with high energy density and high output density, then changed over to middle energy density and middle output density to thereby machine glass, and finally changed over to low energy density and low output density to thereby machine resin.
According to the technique disclosed in JP-A-2002-335063, the pulse frequency and the peak output can be set for each of the copper conductor layer, the glass fiber and the resin. However, when the peak output is reduced while the pulse width is increased, the resin residue at the hole bottom increases. In order to reduce the resin residue, it is therefore necessary to reduce the pulse width while increasing the peak output.